The paradox of sudden slowing of idioventricular rhythm during exercise in complete heart block: Modulation of automatic focus, concealed reentry, or wedensky inhibition?

As a rule, exercise enhances the rate of discharge of idioventricular rhythms in complete atrioventricular (AV) block. The opposite effect, a sudden drop in the rate of discharge with subsequent resetting of the firing rate, was observed in an 18-year-old boy with chronic complete AV block. Two mechanisms for this paradoxical response to exercise are proposed: (1) delay in discharge of the automatic pacemaker focus by successful penetration of electrotonic potentials from nonconducted P waves across the area of block in the AV conduction system and into the pacemaker focus; and (2) discharge of the pacemaker focus by concealed reentrant extrasystoles. Wedensky inhibition is ruled out because of the resetting of the firing rate.